And die in the process due to the lag that occurs whenever you hit that button D:
Unfortunately, there isn't much way to catch these assholes unless you are lucky. Even if you do spam Print Screen, (a)You're likely to die in the process if there's any mob in sight (thus making it hard to prove they were actually in the process of ksing you) and (b)You won't get their name in the picture unless you're lucky enough to have the mouse right over them, which is hard to do, and thus it's impossible to prove the identity of the greeders.
I can't count how many times this has happened to me, with no way of catching it >.> Unfortunately, there's not much that can be done about it. Console yourself with the knowledge that they probably only got a small pile of loots which weren't worth that much anyways, and also with the knowledge that if they ever fuck up and don't flywing in time, they'll be in big trouble and for once everyone can rejoice about it. And there are a few people who are exceptionally good with the print screen, so every now and then these faggots do get caught and the nice lengthy mute they deserve is given to them.